Certain cardiac arrhythmias are triggered or initiated from a site in the heart tissue other than the sinus node. These arrhythmias are classified as being “focal” in nature. Treatment of focal arrhythmias generally involves locating the arrhythmogenic site and ablating it. One method for regionally locating the focal site is the use of a diagnostic 12 Lead ECG. The 12 Lead can be used in conjunction with pacing via a roving intracardiac catheter to pace map the heart. The theoretical basis of this method assumes that the paced 12 lead ECG will appear identical to the non-paced ECG if the cycle length (i.e., paced heart rate) and pacing site matches the non-paced heart rate and focal site of origin.
One problem with this method (in current practice) is the subjectivity involved in visually comparing a non-paced 12 Lead ECG to a paced 12 Lead ECG. The same problem exists when viewing cardiac signals acquired from an intracardiac lead or from a data storage device.
A second problem is the time consuming nature of the procedure in which, typically, a spontaneous ectopic beat is recorded and printed on paper. A roving mapping catheter is positioned at a likely site of ectopy, pacing is initiated, a recording is made, a printout is generated and a visual comparison is made by aligning the printouts from the spontaneous and paced beats over one another. This process is repeated in an iterative manner until the physician determines that a good match between the spontaneous ectopic beat and the paced beat is found.
It will therefore be appreciated that it is a valuable tool for the physician to be able to easily compare components of the ECG (e.g., a beat of interest) with a template reference, such as a closely proximate cardiac signal. The comparison between a data signal (e.g., one cardiac signal) and a template signal (e.g., a closely proximate cardiac signal) can be done in a number of different ways, including the above method of aligning and overlaying printouts; however, these techniques all suffer from disadvantages that make it difficult for the physician to easily make a visual comparison between the printouts.
Moreover, when the cardiac signals are displayed on an electronic display (monitor) as compared to physically overlying printouts, they are typically displayed by placing one of the signals in a first display coordinate while the other signal is displayed in a second display coordinate that is above or below the first display coordinate. In other words, the signals are displayed independent of one another; however, this makes it difficult for the user to easily compare the two signals since the signals are not overlaid on one another and therefore, a visual comparison of different components of the ECG is complex and subject to human error.
As a result, systems have been developed that overlay one cardiac signal on top of the other cardiac signal on the electronic display so that the physician can compare the data signal (one cardiac signal) to the template signal (a template cardiac signal). However, the overlay of the two signals on the display signal creates another disadvantage in that the two signals are depicted on the display in the same color and, therefore, it can be difficult for the physician to distinguish one signal from the other signal. The physician is also interested in ascertaining where there is the best match between the two signals (i.e., the best overlap) and therefore, it is of interest for the physician to clearly see where there is a perfect overlap between the signals or where the two signals closely approximate one another. Unfortunately, the conventional technique of superimposing the signals does not always permit the physician to distinguish between the two signals, especially in the context of considering wave components of a cardiac signal.
The electrocardiogram typically includes an initial impulse, termed the P-wave, emanating from the atria, followed by what is termed the QRS complex, emanating from the ventricles, which is followed by a T-wave resulting from repolarization of the ventricles (FIG. 1). Thus, a heart beat begins with the P-wave and ends with the T-wave, and the next heart beat begins with another P-wave. The P-wave can be a valuable tool used by clinicians to diagnose the condition of the heart. Thus, clinicians will often monitor an electrocardiogram (ECG) of the heart to aid in the diagnosis of atrial and ventricular arrhythmias. This can be done in various ways, a most common technique being by monitoring the 12 Lead (surface) ECG in conjunction with observing the bioelectric activity recorded on intracardiac electrodes carried by a transthoracic catheter.
Accordingly, it will be apparent that there continues to be a need for a method that allows a clinician to pace map more effectively and more specifically, it is desired for a more efficient and effective technique for displaying a data signal (a cardiac signal of interest) over a template signal (e.g., another cardiac signal) and clearly indicating any overlap or close proximity between the two signals.